I wouldn’t hassle with the NameScopes etc. and would rather use Storyboard.SetTarget instead.
var b = new Button() { Content = "abcd" };
stack.Children.Add(b);
var fade = new DoubleAnimation()
{
From = 0,
To = 1,
Duration = TimeSpan.FromSeconds(5),
};
Storyboard.SetTarget(fade, b);
Storyboard.SetTargetProperty(fade, new PropertyPath(Button.OpacityProperty));
var sb = new Storyboard();
sb.Children.Add(fade);
sb.Begin();